The writer to the Hebrews is writing to a group of Jewish people are are seriously considering abandoning the faith and returning to a legalistic form of Judaism. Perhaps they have fallen under the influence of the Essene community, perhaps the persecution of Nero is giving them second thoughts. Either way - they are considering turning back.
The author has reasoned with them in every way imaginable to stay the course… to press on… now he is coming down to his closing arguments. He sums up the argument of chapter 11 in chapter 12.